1. Field of the Invention
The present invention relates to a position control device applied to the axis of rotation of a numerical control machine.
2. Description of the Related Art
FIGS. 3A and 3B are a diagram showing an example of a mechanical model of an axis of rotation that is a control target plant. A rotary table 50 is rotated at an angle of rotation θ about a Zu axis by a servo motor (not shown). The point of intersection between the Zu axis and a rotational locus plane described by a center of gravity G of the rotary table 50 is represented by an origin point Ou, an Xu axis is taken in a perpendicular direction with respect to gravity on the rotational locus plane, and a remaining Yu axis is taken so as to form a right-handed coordinate system with the Zu axis and the Xu axis.
The center of gravity G is at a distance L from the origin point Ou and is in the position of angle a when θ=0. Various types of jigs/tools and workpieces are placed on the rotary table 50 according to respective machining processes, so that the load state changes and the position of the center of gravity G also fluctuates. Note that g represents gravitational acceleration, and angle b is an angle formed between the Yu axis and a plane perpendicular to the direction of gravity.
FIG. 4 is a block diagram showing an example of a conventional position control device 200 for controlling the aforementioned angle of rotation θ of the axis of rotation to a position command value θc generated in a precedence device (not shown).
This device has a feedforward configuration to increase the velocity of command response. Specifically, the position command value θc is temporally differentiated by a differentiator 54 to become a velocity feedforward Vf, and Vf is temporally differentiated by a differentiator 55 to become an acceleration feedforward Af. A gain Cb in an amplifier Cb is a constant that determines an acceleration and deceleration torque feedforward τf corresponding to a motor torque converted to axis of rotation for generating the acceleration Af in the axis of rotation. Ordinarily, Cb corresponds to the sum of the moment of inertia of a transmission system including a motor, which moment of inertia has been axis-of-rotation-converted, and the moment of inertia of the rotary table 50 by itself having no objects placed thereon.
The feedforward configuration of the conventional position control device is as follows. First, the angle of rotation θ detected by an angle-of-rotation detector (not shown) is subtracted from the position command value θc by a subtractor 51, and position error that is the output thereof is amplified by a position error amplifier Gp. Moreover, the output thereof is added to the velocity feedforward Vf by an adder 52 to become a velocity command value Vc. A subtractor 53 subtracts, from the velocity command value Vc, an angle-of-rotation velocity ω in which the angle of rotation θ has been differentiated by a differentiator 56, and velocity error that is the output thereof is ordinarily proportionally integrally amplified by a velocity error amplifier Gv. This output and the acceleration and deceleration torque feedforward τf are added together by an adder 57 to become an axis-of-rotation-converted torque command value τc, which is Ct-amplified by a power amplifier Ct. Ct is a constant determined in accordance with the servo motor characteristic, this output τ becomes the axis-of-rotation-converted generated torque of the servo motor, and the rotary table 50 is driven.
As described above, in the conventional position control device, an overall configuration where feedforward control is added with respect to a nominal linear characteristic is employed to increase the speed of command response as feedback control for compensating for a nonlinear characteristic resulting from gravity and assuring interior stability of the control system. However, as mentioned above, because various types of different jigs/tools and workpieces are placed on the rotary table, the moment of inertia increases over that of the table by itself and the center of gravity changes. When this happens, the acceleration and deceleration torque feedforward τf is insufficient, the feedback control band is reduced, and the nonlinear characteristic resulting from gravity increases, so that controllability deteriorates, leading to a drop in positioning performance and response variations resulting from the positioning angle during positioning.
Further, because these fluctuation factors cannot be grasped, acceleration constraints and velocity constraints resulting from centrifugal force for function-generating the position command value θc have been unable to be judged on the part of the precedence device. As a result, efficient function generation has not been possible. Moreover, during direct drive application that does not have a deceleration mechanism, the aforementioned fluctuation factors experience a relative increase, tending to make these problems even greater.